Add SONAME build option to facilitate building for Android.

Unfortunately, [Android does not support versioned libs](http://www.opengis.ch/2011/11/23/creating-non-versioned-shared-libraries-for-android/). Since there is no option to disable this during building, I have added an additional option (`SONAME`) to prevent `VERSION` and `SOVERSION` from being set for the `git2` target.

This new option is `ON` by default but can be turned off when building the library for Android. I have tested this change locally and can confirm that non-versioned libraries are produced when this option is set to `OFF`.